1. Field of the Invention
The present invention relates generally to X-ray apparatus and more particularly to a collimator structure therefor. Specifically, the invention is concerned with an apparatus which automatically controls the width and thickness of X-ray beams in X-ray diagnostic equipment, and is particularly adapted for use with a device known as a computerized axial tomographic scanner.
2. Description of the Prior Art
Collimators for controlling the size and shape of X-ray beams are well known in the art and include those shown in Haupt U.S. Pat. No. 2,542,196, Akaski et al U.S. Pat. No. 2,851,610 and Hura U.S. Pat. No. 3,829,701.
Mechanisms known as collimators are commonly used to shape an X-ray beam to a desired size and shape. In equipment used in radiographic studies, these collimators generally include two pairs of relatively movable diaphragms which shape an X-ray beam so as to provide a rectangular cross-section. These diaphragms are also used to eliminate two sources of extraneous radiation which may cause a degradation of the image produced by the diagnostic equipment.
A first cause of extraneous radiation is commonly referred to as scatter. In radiographic studies, it is desirable to confine the X-ray beam to the area of the object under examination, not only to minimize the exposure of the object and attending persons to the primary beam, but also to minimize radiation scatter effects. Radiation scatter is produced when the primary radiation beam strikes an object and is defracted. If the size of the X-ray beam is larger than required to accommodate a particular area under investigation, the X-ray beam striking areas of the object around the area of investigation will produce an unnecessary amount of radiation scatter which has the effect of reducing the contrast of the radiographic image.
A second source of extraneous radiation which may cause image degradation is caused by what is known as the "penumbra" effect. The X-ray beams are emitted from a very small area on an X-ray tube anode known as the focal point. Theoretically, this spot can be so small and bombardment of it with electrons so precise that the beam is emitted in a precise and regular conical pattern of "on focus" radiation. As a practical matter, however, the spot is a larger area than a theoretically optimized spot and an X-ray tube emits a penumbra or band of so-called "off focus" radiation from areas around the spot. This penumbra or "off focus" radiation is another source of image degradation.
Computerized axial tomographic X-ray scanners which provide the reconstruction of a transaxial section of an object by means of X-rays are also well known in the art as evidenced by Hounsfield U.S. Pat. No. 3,778,614. A reconstructed image of an object can be obtained by viewing an object via X-ray imaging from numerous angles, mathematically reconstructing the detailed structure, and displaying the reconstructed image. In general, X-ray beams are passed through the object for detection by scintillation crystal detectors. Analog outputs from these detectors go through signal conditioning circuitry that amplifies, clips and shapes the signals. A relatively simple analog to digital converter then prepares the signals for the computer, which performs various mathematical operations upon the data received and provides an output which may be used to display the reconstructed image.
Prior art computerized axial tomographic scanners have generally utilized fixed type collimators as evidenced by Hounsfield U.S. Pat. No. 3,778,614, the disclosure of which is incorporated by reference. In the case of a computerized axial tomographic scanner having a plurality of radiation detectors, the conventionally used collimator structure requires a rather complex apparatus which is expensive to fabricate and greatly increases the bulk of that portion of the scanner which is required to be moved, with the attendant disadvantages.
Even further, the conventional collimator structure is difficult to use because of its inflexibility. This inflexibility becomes readily apparent when it is desired to use a different size or shape X-ray beam during the scanning process. In order to realize this feature with the conventional collimator structure, it is necessary to entirely remove the collimator structure and replace it with a collimator structure producing the desired size or shape of X-ray beam. The expense and inconvenience of such a procedure often makes it unfeasible to effect a change in the size or shape of the X-ray beam used by the scanner.
There is, therefore, a need for a computerized axial tomographic scanner having a variable collimator means which may be used to readily change the size or shape of the X-ray beam utilized by the scanner, which change may be accomplished with relative ease and in a short period of time, and which provides a collimator means that minimizes extraneous radiation in order to enhance image quality.
The present invention solves this problem by providing an automatic variable collimator structure which utilizes a two-part collimator structure having movable aperture plates therein to effectively change the size and shape of the X-ray beams utilized by the scanner. The collimator structure additionally minimizes extraneous radiation.